The present invention relates to noise absorbing devices or absorption elements for the damping of noise. A plurality of such devices are arranged on the walls and/or ceilings of halls, rooms, passageways, and channels for soundproofing such spaces.
In order to achieve high noise absorptions which are necessary, for example, in so-called anechoic chambers, it is known to use so-called .lambda./4 wedges or so-called Cremer's absorption cubes. Such cubes and wedges are made of absorption material, for example, open cell synthetic foam or rock wool. Both prior art systems provide a sufficient noise absorption above the so-called cut-off frequency. However, the absorption disappears below said cut-off frequency. The cut-off frequency is determined by the length of the wedges which in turn determine the structural depth of the soundproofing means. By "structural depth" in this context is meant the space taken up between a wall of a room and the front face of the soundproofing means facing into the room. Conventionally the structural depth corresponds to 1/4.sup.th of the cut-off wave length .lambda.. For example, for a cut-off frequency of 100 Hz the respective wave length in air corresponds approximately to 3.4 m, so that the necessary structural depth corresponds to 3.40.div.4=0.85 m. This is a substantial space requirement and can frequently not be satisfied, especially by smaller rooms. Although it is possible to lower the effective cut-off frequency with the aid of Helmholtz-Resonators, substantial structural depths are nevertheless still required whereby the available useful space is respectively made smaller.
It is also known to employ absorption mats or blankets as wall and ceiling covers in rooms which do not require the high noise absorption which is required in so-called anechoic chambers. In this type of arrangement the spacing between the absorption mat or blanket and the wall again determines the absorption factor. Mats or blankets which are secured to the wall without intermediate space are not capable of damping the lower and intermediate frequencies. As a result, even such mats or blankets also require relatively large structural depths comparable to the above mentioned .lambda./4 condition.